[SLP] Fix regression in broadcasts caused by operand reordering patch D59973.

    
This patch fixes a regression caused by the operand reordering refactoring patch https://reviews.llvm.org/D59973 .
The fix changes the strategy to Splat instead of Opcode, if broadcast opportunities are found.
Please see the lit test for some examples.

Committed on behalf of @vporpo (Vasileios Porpodas)
    
Differential Revision: https://reviews.llvm.org/D62427

llvm-svn: 362613